1. Field of the Invention
This invention relates to electron beam focussing in X-ray apparatus.
The invention is applicable for example to X-ray apparatus requiring a point source of X-rays. Focussing of an electron beam is required in for example, X-ray microscopes, employing a point focus X-ray projection technique; an electron beam from a gun in such apparatus is focussed onto a target of a suitable metal to emit X-rays. For microscopy, it is essential that the electron beam must be sharply focussed to give a substantially point source of the X-rays. With point focus projection techniques, by disposing a specimen to be examined between the point source of X-rays and a fluorescent viewing screen or other viewing device or photographic emulsion, magnification is obtained. Further magnification can be obtained by projecting the image onto the input phosphor of an X-ray sensitive camera and viewing a television monitor connected to the camera.
2. Prior Art
It is a well-known practice in X-ray microscopes to employ a sharply bent hairpin filament arranged so that the sharp bend in the filament is the part thereof nearest the target. The electrons are emitted from the tip of such a filament and are partially focussed by a biased grid cylinder between the filament and the target. This grid cylinder is normally cup-shaped and has a diaphragm with a slit or aperture. It has to be accurately positioned with respect to the filament and electrically biased with respect thereto. For electrostatic focussing, there may be provided a further focussing electrode which is suitably biased and may take the form of a ring or cylinder or a cup. Alternatively, for electromagnetic focussing, further focussing means incorporating a coil may be provided. The electron beam is focussed onto the target, which is of a suitable material, such as tungsten, which will emit X-rays under the electron beam bombardment. It is convenient to use a cylindrical target and the electron beam is focussed at a suitable point on the cylinder such that the X-rays are emitted outwardly at a direction at an angle to the axis of the electron beam. By using a cylindrical target which is made rotatable and slidable about its axis, if a point on the target becomes pitted by the electron beam, the target may be rotated and/or moved axially to bring a fresh portion of the target material into the focus position of the electron beam while still leaving a target surface at the same angle.
In my British Patent Specification No. 1250007 there is described an electron beam equipment having a demountable electrode assembly including a hollow support cylinder adapted to extend inwardly of an equipment envelope to be withdrawable therefrom and to be fastened in vacuum-sealing engagement with the envelope, a detachable cap at the inward end of the cylinder adapted to be fitted to it in vacuum-sealing engagement, various electrode components supported by said cap, at least one of said components being adjustable relatively to the others, actuator means for said adjustable component or components extending through and adjustable relative to the cap and in vacuum-sealing engagement therewith, and adjusting means extending lengthwise within the hollow cylinder and coupled to the actuator means for selective displacement thereof, said adjusting means being disposed parallel to an electric supply cable or to electric supply conductors extending within the cylinder and coupled to the component supported by the cap, means being included for imparting displacement to said adjusting means and to said actuator means. The electrode assembly more particularly comprises an adjustable electron gun having a filament supported on the cap and surrounded by an apertured diaphragm having a small hole or slot therein and at least one focussing cup, the diaphragm and the cup or cups being adjustable by independent adjusting means consisting of rods with independent means for displacing the said rods. Problems with this form of construction arise from the need to prevent any voltage breakdown between the components and more particularly because of the inability to provide adequate orthogonal positioning of the auxiliary focussing means.